An essential component of most modem electronic appliances, including computers, is a disk drive for storing information on magnetic disks or platters. In a hard disk, magnetic recording material is layered onto a high-precision platter (disk) (usually made of glass or aluminum). Typically, the information is stored in concentric tracks usually divided into sectors.
The disk drive includes a reading and recording head (head), a positioning component and a controller. A typical positioning component includes an actuator arm to position the head on the right track on the magnetic platter and a track following system that keeps the head in place.
The actuator arm is controlled by the controller, so that the head, in an initial position, is moved to a target track position. That is referred to as the “seek” phase.
In the seek phase, the actuator arm goes through five phases. In the “speedup” phase the actuator arm is accelerated until it reaches half of the seek distance or a fixed maximum velocity. The actuator arm then enters the “coast” phase where the actuator arm keeps moving at a steady speed. The actuator arm the enters the “slowdown” phase where then the actuator arm is brought to rest very near to the target track. The final phase is the “settle” phase where the controller adjusts the head to access the target track. Once the head is on the right track the controller keeps the head on the desired track until the completion of the data transfer. That is referred to as the “tracking” phase.
During the settling phase the head will oscillate on the desired frack for some time as the controller transitions from the seek phase to the tracking phase. Data written while the head is oscillating may not be reliably stored thereby making the data unreadable . To avoid this problem, the controller prevents a write operations for a predetermined time after the transition from the seek mode to the track following mode.
Similarly, a reading operation may be prevented for some time following the transition from seed to track to avoid data from adjacent tracks being inadertently read. The delay of such read and write operations decreases th ferformance of the disk druve.
The inefficiency is also seen in erasure-encoded disks during read-before-write and read-modify-write poerations. In a typical disk drive, the operationsare more time consuming than a write operation by nearly a full rotation of the platters, which, for current disks 3 to 12 milliseconds. For many applications this insighficant but for others the amount of buffering required to hide latency would be onerous.
As an example, consider a computer calculation of a parity value: the “exclusive-or ”(a Boolean operator that returns a value of TRUE only both its operands have different value) of data values A and B. Each write to the platter is best done by reading the previous value, writing the new value, then computing D, the difference (i.e. exclusive-or) between the old and new values. To maintain a parity value, we must then replace the previous parity value with the difference between its old value and D.
In a typical disk drive, the positioning component will move the head into the right general neighborhood of the track currently being read or written, and then make fine adjustments to the head position until it is actually above the correct track. Tn the case of a read operation, once the data on the surface can be resolved to be the correct sector of the correct track, the result can be returned; the low-level error-correcting cpdes allow the platter to verify that the bits that have been read all belong to the desired sector. For a write operation, the process takes more time (about 2 milliseconds), because the position of the disk head continues to oscillate slightly. Performing a write operation during these oscillations could be problematic: the data bits written might belong to a sector on a neighboring track, whose value would then be corrupt. While the low-level error-correcting codes on a platter can cope with a small number of such miswrites, a large number would result in the loss of the data from the neighboring sector. As disks become smaller and track densities increase, the fraction of the total seek time attributed to the settle phase increases.